This section provides background information related to the present disclosure which is not necessarily prior art. This section further provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Around the world, civil infrastructures such as buildings, bridges, lifelines, among others, represent the foundation for economic welfare and societal prosperity. Many of these vital structures are beginning to approach (or have already exceeded) their design service lifetimes. Today, $91 billion is spent annually to maintain the U.S. inventory of highways and bridges; however, an additional $128 billion is needed to upgrade existing structures to current standards. As such, efficient and cost-effective strategies are required to ensure infrastructure serviceability and safety. In most cases, the current state of practice relies on schedule-based maintenance routines in which engineers rely on visual inspection to assess structural performance. Not only is this method subjective, but a schedule-based maintenance program is often economically inefficient, as newer structures may not need inspection during their initial years of service.
As a result, many researchers have proposed tethered sensor networks for monitoring structural performance over time, commonly termed structural health monitoring (SHM). Using a few distributed sensors (e.g. accelerometers) installed within the civil infrastructure then coupled with automated damage detection algorithms at the centralized data repository, a comprehensive SHM system can be formed. Structural health monitoring can objectively monitor long-term structural reliability and serviceability. However, the high costs to install and maintain the extensive coaxial cables connecting sensors to the centralized data repository have warranted novel cost-effective methods for SHM. For example, the cost to install tethered sensors in tall buildings and long bridges can exceed thousands of dollars on a per channel basis. High system costs result in low sensor densities in large-scale civil infrastructures; as a result, generally only global vibration characteristics are deduced from so few sensors. Nevertheless, the advent of tethered sensors for SHM has initiated the shift from schedule-based to performance-based monitoring.
Instead of using cable-based sensors for global vibration structural characterization, a variety of academic and commercial wireless sensor networks have been proposed for densely distributed SHM systems. Costing approximately $100 per sensing node, low wireless sensor costs permit high nodal densities for component-level damage detection (e.g. monitoring strain and corrosion processes). Furthermore, with local computational power embedded within each sensor node, distributed data processing and wireless structural control have been achieved. Numerous field validation studies conducted with wireless sensors have indicated performance levels comparable with traditional cable-based monitoring systems. Unfortunately, one significant disadvantage of the aforementioned wireless sensors is their inherent dependency on power supplies (e.g. batteries or AC power source). To conserve power, some researchers have adopted trigger-based power-on mechanisms (i.e. when acceleration exceeds a preset threshold) as well as local data processing to solely transfer computed results (as opposed to the entire time history record) to reduce power consumed by the wireless transceiver. These efforts have only led to moderate improvements in sensor service lifetimes with life expectancy to approximately two years. Furthermore, while methods for converting ambient mechanical vibration into electrical energy are currently underway, the field of power harvesting is still in its infancy.
In order to preserve the advantages offered by wireless sensing while simultaneously addressing issues regarding power limitations, some researchers have adopted inductively coupled radio frequency identification (RFID) sensing systems for strain and corrosion monitoring. Through the use of a coil antenna wirelessly coupled to an AC (alternating current) generator (i.e. the reader), one can inductively power and communicate with a remote passive sensor circuit in close proximity. Early investigatory work in RFID sensing has been proposed where they have developed a passive wireless peak strain sensor based on two concentric aluminum pipes sliding over a dielectric material. Upon installing these sensors to the base of a seven-story base-isolated building, experimental peak strain data collected from the prototype RFID sensor coincides with those obtained from a laser displacement transducer. Extension to this work seeks the utilization of MEMS (microelectromechanical systems) processes to miniaturize the capacitive peak strain sensor. As opposed to measuring peak strain, some systems have developed a passive thick film strain sensor by incorporating poly(vinyl fluoride) with an interdigital capacitor to enhance the sensitivity of characteristic frequency shifts to strain. On the other hand, for monitoring corrosion processes, some systems have developed a 2.4 GHz RFID wireless sensor to detect the loss of interfacial bond strength and reduction in steel-reinforcement cross-sectional area in concrete via acoustic emissions. To accurately monitor different thresholds of concrete corrosion wirelessly, some systems utilize an exposed switch fabricated with different gauge steel wires. When corrosion destroys the exposed wire switch, dramatic characteristic frequency shifts have been observed between initial and corroded states. Unfortunately, among the wide variety of RFID-based strain and corrosion sensors that exist, most have a large form factor and are derived by miniaturizing mechanical elements.
According to the principles of the present teachings, a prototype thin film passive wireless strain and pH sensor is provided for localized strain and corrosion monitoring. Encoding of electromechanical and electrochemical sensing transduction mechanisms (i.e. strain and pH, respectively) within a thin film structure is accomplished by adopting material fabrication techniques derived from the nanotechnology domain. Nanotechnology provides tools and materials such that, by manipulating material properties at the molecular scale, one can utilize a “bottom-up” design methodology to yield high performance sensors. In particular, single-walled carbon nanotubes (SWNTs) and a variety of polyelectrolyte (PE) species combined with a layer-by-layer (LbL) fabrication technique can produce a homogeneous multilayer thin film sensor of controlled morphology. When coupled with a coil antenna, the final multifunctional sensor package is capable of wirelessly detecting strain and pH via characteristic frequency and bandwidth changes, respectively.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.